In the development of electroluminescent devices utilizing organic materials, optimization of the kind of electrodes for the purpose of improving the charge-injecting efficiency from the electrodes and disposition of a hole-transporting layer of an aromatic diamine and a light-emitting layer of 8-hydroxyquinoline aluminum complex (hereinafter referred to as Alq3) in thin film between the electrodes have created a device with a remarkable improvement in luminous efficiency over the conventional devices that utilize single crystals of anthracene and the like. Following this, the developmental works of organic EL devices have been focused on their applications to high-performance flat panels characterized by self-luminescence and high-speed response.
In an attempt to improve the luminous efficiency of such organic EL devices still further, modifications of the aforementioned basic structure of anode/hole-transporting layer/light-emitting layer/cathode by suitable addition of a hole-injecting layer, an electron-injecting layer, or an electron-transporting layer have been found effective for enhancing the luminous efficiency and a large number of organic materials conforming to the function of these layered structures have been developed.
In another attempt to enhance the luminous efficiency of an organic EL device, the use of phosphorescence in place of fluorescence has been investigated. The aforementioned device comprising an aromatic diamine in the hole-transporting layer and Alq3 in the light-emitting layer and many other devices utilize fluorescence. Now, the utilization of phosphorescence, that is, emission of light from the excited triplet state, is expected to enhance the luminous efficiency approximately three times that of the conventional devices utilizing fluorescence (singlet). The prior documents relating to this invention are listed below.
Patent document 1: WO00/70655
Patent document 2: JP2001-284056A
Patent document 3: JP5-198377A
Patent document 4: JP2003-142264A
Patent document 5: WO2002/47440
Patent document 6: WO001/041512
Patent document 7: JP2001-313178A
Patent document 8: JP2002.305083A
Patent document 9: JP5.214332A
Non-patent document 1: Appl. Phys. Lett., Vol. 77, p 904 (2000)
Reports are published in recent years on the possibility of enhancing the luminous efficiency in phosphorescent electroluminescence by doping the light-emitting layer with an iridium complex as a guest material and a number of disclosures are made in the patent documents 1 and 6 and elsewhere. A typical example is tris(2-phenylpyridine)iridium complex (hereinafter referred to as Ir(ppy)3) which is a phosphorescent material emitting green light. It has been found that iridium complexes are made to emit light in a wide wavelength range from blue to red by changing the chemical structure of the ligands.
The use of 4,4′-bis(9-carbazolyl)biphenyl (hereinafter referred to as CBP) as a host material in the light-emitting layer of an organic EL device is proposed in the patent documents 1 and 7. However, CBP has a specific property of facilitating the flow of holes and obstructing the flow of electrons and CBP used as a host material for Ir(ppy)3 destroys the balanced injection of electrical charges thereby causing excess holes to flow out to the side of the electron-transporting layer. As a result, the luminous efficiency from Ir(ppy)3 drops.
One of the means to solve the aforementioned problems is providing a hole-blocking layer between the light-emitting layer and the electron-transporting layer as described in the patent documents 2 and 8. The hole-blocking layer can attain the object of enhancing the luminous efficiency by accumulating holes effectively in the light-emitting layer and improving the probability of recombination of holes with electrons in the light-emitting layer. The hole-blocking materials in general use include 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (hereinafter referred to as BCP) and p-phenylphenolato-bis(2-methyl-8-quinolinolato-N1,O8)aluminum (hereinafter referred to as BAlq). The hole-blocking layer thus provided can prevent electrons and holes from recombining in the electron-transporting layer. However, a device utilizing BCP shows an extremely short life as BCP tends to crystallize easily even at room temperature and lacks reliability as a hole-blocking material. On the other hand, BAlq has an insufficient ability to block holes and causes a drop in the luminous efficiency from Ir(ppy)3, although it is reported to show a relatively satisfactory life. Moreover, providing the hole-blocking layer means adding one more layer which complicates the structure of a device and increases the cost.
On the other hand, the use of the aforementioned BCP and 3-phenyl-4-(1′-naphthyl)-5-phenyl-1,2,4-triazole (hereinafter referred to as TAZ) as a host material in phosphorescent organic EL devices is proposed; however, the cited compounds have a specific property of facilitating the flow of electrons and obstructing the flow of holes and their use as a host material shifts the light-emitting range toward the side of the hole-transporting layer. Therefore, there may arise a problem of the luminous efficiency from Ir(ppy)3 dropping depending upon the compatibility of Ir(ppy)3 with the material chosen for the hole-transporting layer. For example, 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (hereinafter referred to as α-NPD) is used most widely as a material for the hole-transporting layer on account of its good performance, high reliability, and long life; however, the use of α-NPD together with Ir(ppy)3 causes transition of energy from Ir(ppy)3 to α-NPD and results in a drop of the luminous efficiency.
A BAlq-containing luminescent composition emitting blue light is disclosed in the patent document 9. As is apparent here, BAlq and related compounds are used not only as light-emitting materials but also as materials for other layers.
It is reported in the non-patent document 1 that light can be emitted at high efficiency from a phosphorescent electroluminescent device of three-layer structure constituted of a light-emitting layer containing TAZ or the like as a host material and Ir(ppy)3 as a guest material, an electron-transporting layer containing Alq3, and a hole-transporting layer containing 4,4′-bis[N,N′-(3-toluyl)amino]-3,3′-dimethylbiphenyl (hereinafter referred to as HMTPD). However, HMTPD tends to crystallize easily as its glass transition temperature (hereinafter referred to as Tg) is approximately 50° C. and lacks reliability as a hole-transporting material. In consequence, a device of the aforementioned structure encounters problems in that it shows an extremely short life, it is not readily applicable commercially, and it requires high driving voltage.
The patent document 3 discloses the incorporation of a dimeric metal complex containing an 8-quinolinol ligand represented by Q2-Al—O—Al-Q2 in the blue light-emitting layer and the use of this complex together with a fluorescent colorant such as perylene and the patent document 4 discloses the use of a dimeric metal complex as a phosphorescent host material; however, these patent documents do not teach the usefulness of a deuterated dimeric metal complex. Here, the dimeric metal complex refers to a metal complex having a structure represented by Q2-Al—O—Al-Q2 wherein Q is a substituted or unsubstituted 8-quinolinol ligand.
Isotopic atoms such as 2H (termed heavy hydrogen or D) and 13C have been utilized widely for labeling with isotope tracers in medical treatment and structural analysis of compounds. In connection with the organic EL field, the patent document 5 discloses that the carbon-deuterium (C-D) bond is shorter than the carbon-hydrogen (C—H) bond and the former is more stable physicochemically than the latter and cites a variety of deuterated compounds (designated as compound-D).
Hetero ligand metal complexes and dimeric metal complexes such as BAlq are useful as organic EL materials; however, none of the documents teaches the necessity or effectiveness of replacing hydrogen atoms in the methyl group at position 2 (benzylic hydrogen) in the quinolinol ligand with deuterium atoms.